Hormones: types, receptors and feedback !

Concept
Exam priority: medium

Hormones are chemical messages in the blood. Protein hormones act at the membrane; steroid and thyroid hormones act inside the cell.

Updated 2026-11-01

Also called: Hormone classes, Negative and positive feedback

What it is

Hormones are grouped by their chemistry, and the chemistry predicts nearly everything else: how they are stored and carried, where their receptor sits, how fast they act and whether they work by mouth. Almost every axis is run by negative feedback, which holds a set point; positive feedback is the exception and always has an end. Signal pathway details are in Biochemistry.

Key facts

Membrane receptors: Gs–cAMP (glucagon, ACTH, TSH, PTH, ADH at V2), Gq–IP₃/Ca²⁺ (ADH at V1, oxytocin, GnRH, TRH), tyrosine kinase (insulin, IGF-1), JAK-STAT (GH, prolactin).
Many hormones are secreted in pulses and with a circadian rhythm: one value may not represent the average.
Down-regulation (fewer receptors) explains why continuous GnRH shuts the gonadal axis down while pulsatile GnRH drives it.
Feedback loops: long (final hormone → hypothalamus and pituitary), short (pituitary hormone → hypothalamus).

Hormone axes and feedback

Hypothalamic–pituitary–thyroid axisLesion None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

Stimulates (+)
Inhibits / negative feedback (−)
Level ↑
Level ↓

Illustrative schematic drawn by Durgical: simplified axes and classic lab patterns; not for clinical decisions.

Hormone classes

Feature	Peptides and proteins	Steroids	Amines
Examples	Insulin, glucagon, PTH, ADH, pituitary hormones	Cortisol, aldosterone, estradiol, testosterone, calcitriol	Catecholamines (like peptides); T3/T4 (like steroids)
Storage	Secretory granules	Not stored	Catecholamines in granules; T4 in thyroid colloid
Transport in blood	Mostly free	Protein-bound	Catecholamines free; T4/T3 bound
Receptor	Membrane (Gs, Gq, tyrosine kinase, JAK-STAT)	Intracellular (nuclear)	Catecholamines: membrane; T3: nuclear
Speed and duration	Fast and brief	Slow and lasting	Depends on the subclass

Clinical notes

Peptide hormones are digested in the gut and so are not given by mouth (insulin); steroid and thyroid hormones are absorbed.
Changes in binding proteins (pregnancy, cirrhosis, nephrotic syndrome) alter the total level without changing the free fraction.

Test yourself

Think of the answer, then tap the question to see it.

Why does cortisol secretion equal its synthesis, while insulin can be released in a burst?

Steroids are lipid-soluble and cross membranes, so they cannot be kept in vesicles; insulin, a peptide, is stored in granules ready to go.

Why does continuous GnRH shut the gonadal axis down while pulsatile GnRH stimulates it?

Continuous exposure desensitizes and down-regulates GnRH receptors on gonadotrophs; pulses leave recovery time between stimuli.

Why does thyroid hormone, an amine, behave like a steroid?

It is lipid-soluble, travels on proteins, enters the cell and acts on a nuclear receptor that changes transcription, with slow and lasting effects.

What feature marks positive feedback in physiology?

It amplifies the change instead of correcting it and needs an event to end it: ovulation ends the LH surge, and delivery of the fetus ends the oxytocin reflex.

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In Biochemistry

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 17.2 Hormones (OpenStax (Rice University), 2022); 17.1 An Overview of the Endocrine System (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 75). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.